This invention relates to modulation systems and methods, and more particularly to dual-mode modulation systems and methods.
Modulation systems and methods are widely used in transmitters to modulate an information input including voice and/or data onto a carrier. The carrier may be a final carrier or an intermediate carrier. The carrier frequency can be in UHF, VHF, RF, microwave or any other frequency band. Modulators are also referred to as xe2x80x9cmixersxe2x80x9d or xe2x80x9cmultipliersxe2x80x9d. For example, in a mobile radiotelephone, a modulator is used for the radiotelephone transmitter.
As is well known to those having skill in the art, modulation systems and methods for digital input signals generally include a Digital-to-Analog Converter (DAC) that converts the digital input signal into an analog signal. A low pass filter, also referred to as an xe2x80x9canti-aliasing filterxe2x80x9d, filters the analog signal to produce a filtered analog signal. A modulator modulates the filtered analog signal onto a carrier. The modulator includes a multiplier that is coupled to a local oscillator, such as a Voltage Controlled Oscillator (VCO), and to the filtered analog signal. The carrier including the filtered analog signal may then be transmitted by an antenna.
In modern communications systems, it is often desired to provide dual-mode modulation systems and methods that can modulate two types of communications signals. For example, in mobile radiotelephones, it is often important to provide a modulator that operates both in narrowband FM mode and in wideband Code Division Multiple Access (CDMA) mode. More particularly, in order to provide a mobile radiotelephone that can be used with both an IS-19 AMPS analog system and an IS-95 Direct Sequence Spread Spectrum (DSSS) wideband CDMA system, it is desirable to provide dual-mode modulation systems and methods.
Unfortunately, it may be difficult to provide a dual-mode modulation systems and methods that can handle the disparate bandwidths of the AMPS and CDMA signals. In particular, the narrowband AMPS FM signal has a bandwidth of about 12.5 KHz, while the wideband CDMA signal has a bandwidth of about 615 KHz, or about an order of magnitude wider.
In modem radiotelephone communications, mobile radiotelephones continue to decrease in size, cost and power consumption. In order to satisfy these objectives, it is generally desirable to share circuitry in dual-mode radiotelephones. Shared circuitry can decrease the number of components that are used in the modulator, thereby allowing a decrease in the size thereof. Shared components can also decrease the power consumption of the dual-mode modulation system, which can allow an increase in battery time. Finally, sharing of components can allow a decrease in component cost, thereby allowing a decrease in the overall cost of the radiotelephone.
FIG. 1 illustrates a first conventional dual-mode modulator. As shown in FIG. 1, an IQ modulator 10, also referred to as a xe2x80x9cquadraphase modulatorxe2x80x9d or a xe2x80x9cquadrature modulatorxe2x80x9d includes a quadrature splitter 20, also known as a 90xc2x0 phase shifter, and a pair of multipliers 16a, 16b coupled to the quadrature splitter. A local oscillator 15, such as a Voltage Controlled Oscillator (VCO), is coupled to the quadrature splitter 20 to produce 90xc2x0 phased shifted local oscillator signals. I data 11a and Q data 11b are coupled to a respective multiplier or mixer 16a, 16b respectively. Digital input data is converted to analog data by I Digital-to-Analog Converter (DAC) 14a and Q DAC 14b, respectively. The outputs of the DACs 14a and 14b respectively are applied to low pass filters 12a and 12b respectively to provide the I and Q data inputs 11a and 11b respectively. The modulator modulates the input data on a carrier 13, by summing the outputs of the multipliers 16a, 16b at summing node 218, and transmits the modulated carrier 13 via an antenna.
The DACs 14a and 14b, low pass filters 12a and 12b and IQ modulator 10 may be used to modulate a high bandwidth CDMA signal such as a Direct Sequence Spread Spectrum (DSSS) signal onto a carrier. Since the signal is generated digitally, it is low pass filtered by filters 12a and 12b to let the information through while removing digitally generated spurs and noise.
In order to use the IQ modulator 10 of FIG. 1 in a dual-mode, such as for narrow bandwidth FM signal, a separate FM DAC 19 and a separate FM low pass filter 17 may be provided. Baseband circuitry generates an FM voltage signal that is applied to the tune line of the VCO, to modulate the FM information onto the carrier for transmission according to the AMPS standard. Since the FM voltage signal is generated digitally, it is low pass filtered by FM low pass filter 17 to let the information through while removing digitally generated spurs and noise.
The low pass filter 17 generally has a different bandpass characteristic than the low pass filters 12a and 12b that are part of the CDMA modulator, due to the widely differing bandwidths of the FM and CDMA signals. Accordingly, in this dual-mode embodiment, a separate FM DAC 19 and a separate FM low pass filter 17 is provided. Modulation systems according to FIG. 1 have been designed into many integrated circuit chip sets developed for CDMA standards that also include AMPS functionality. Unfortunately, this technique uses separate DACs and low pass filters, which may increase the size, cost and/or power consumption of the modulator.
A second dual-mode modulation system is illustrated in FIG. 2. In this figure, an IQ modulator 210 including a quadrature splitter 220, a pair of multipliers 216a and 216b, a summing node 218 and a VCO 215 are provided to produce a modulated carrier 213. However, in contrast with FIG. 1, the DACs and low pass filters are shared for the dual-mode operation. In particular, the I DAC and Q DAC 214a and 214b respectively are used for both wideband CDMA and narrowband FM operation. Low pass filters 212a and 212b are also used for wideband CDMA and narrowband FM operation.
Unfortunately, due to the widely disparate bandwidths of the CDMA signal and the FM signal, the low pass filters 212a and 212b should have different band pass characteristics when in the different modes. In order to share the low pass filter, the band pass frequency is switched depending upon mode. Accordingly, while these switched filters 212a, 212b are used in both modes, they may be expensive to implement and may consume excessive power and/or area in a radiotelephone.
In high performance communications systems, it also may be desirable to provide high carrier suppression. In order to provide high carrier suppression, a low DC offset should be produced in the modulation system. For example, the required carrier suppression for FM modulation in an IS-19 AMPS analog system may be approximately xe2x88x9235 dBc. In order to provide an acceptable design margin, it may be preferred for the nominal carrier suppression to be xe2x88x9240 dBc, which can translate into a 14 mV differential DC offset signal when a 2V peak-to-peak differential information signal is generated in a balanced system.
Low DC offset in the digital input signal may be provided using conventional techniques. Unfortunately, however, the modulation system may generate its own DC offset. More specifically, the digital-to-analog converter and/or the low pass filter may generate DC offsets.
The DC offset that is generated in the digital-to-analog converter can be reduced using high performance digital-to-analog converters. Unfortunately, these digital-to-analog converters may be costly and complex. DC offset can be reduced in the low pass filter by providing a passive, off-chip filter with tight tolerance components. Unfortunately, such a passive off-chip filter may be costly and complex, and may consume excessive space in a portable radiotelephone.
It is therefore an object of the present invention to provide improved dual-mode modulation systems and methods.
It is another object of the present invention to provide dual-mode modulation systems and methods for a first signal and a second signal of narrower bandwidth than the first signal.
It is still another object of the present invention to provide dual-mode modulation systems and methods for a first signal and a second signal of narrower bandwidth than the first signal, that can share components of the modulation system to provide compact, low cost and/or low power dual-mode modulation.
It is yet another object of the present invention to provide dual-mode modulation systems and methods that can generate low DC offset.
These and other objects are provided, according to the present invention, by modulating a narrow bandwidth signal, such as a narrowband FM signal, in a modulator that modulates a wide bandwidth signal, such as a CDMA signal, by oversampling the narrow bandwidth signal and applying the oversampled narrow bandwidth signal to the modulator. By oversampling the narrow bandwidth signal, the same fixed low pass filter can be used for both the wide bandwidth signal and the oversampled narrow bandwidth signal. Accordingly, different low pass filters or switched low pass filters are not needed.
In a particular aspect of the present invention, a CDMA modulator including a sampler is used for dual-mode modulation by applying a narrow bandwidth FM signal to the CDMA modulator, such that the CDMA modulator oversamples the FM signal and modulates the oversampled FM signal. The CDMA modulator includes a fixed low pass filter having a passband that encompasses a CDMA signal and the oversampled FM signal, so that the same fixed low pass filter is used to filter both the CDMA signal and an FM signal. The CDMA modulator may be particularly useful in a radiotelephone where the CDMA signal may be a direct sequence spread spectrum signal and the FM signal may be an analog cellular telephone signal.
Dual-mode modulation systems according to the present invention include means for modulating an applied signal onto a carrier and means for applying a first signal to the modulating means, to thereby modulate the first signal onto a carrier. Oversampling means is included for oversampling a second signal of narrower bandwidth than the first signal. The systems also include means for applying the oversampled second narrower bandwidth signal to the modulating means, to thereby modulate the second narrower bandwidth signal onto a carrier.
The modulating means preferably comprises a digital-to-analog converter and a low pass filter that filters the analog output of the digital-to-analog converter, wherein the low pass filter has a passband that encompasses the first signal, and the oversampled second narrower bandwidth signal, such that the same fixed low pass filter is used to filter both the first signal and the oversampled second narrower bandwidth signal. When the modulating means comprises an IQ modulator having I and Q inputs, the oversampling means preferably comprises first and second samplers.
Dual-mode modulation systems according to the invention also include means for sampling an applied signal, means for converting the sampled signal to an analog signal, means for low pass filtering the analog signal and means for modulating the low pass filtered analog signal onto a carrier. Dual-mode modulation systems also include means for applying a first signal to the sampling means, to thereby modulate the first signal on a carrier using the sampling means, the converting means and the low pass filtering means, and for applying a second signal of narrower bandwidth than the first signal to the sampling means, to thereby oversample the second signal and modulate the second signal on a carrier using the sampling means, the converting means and the low pass filtering means. Accordingly, the same unswitched filters may be used for both the wide and narrow bandwidth signals, to thereby allow reduction in cost, space and/or power consumption.
Dual-mode modulation systems and methods according to the invention also compensate for the DC offset that is introduced by the digital-to-analog converter and/or the low pass filter thereof. Compensation is preferably provided in the digital domain, to thereby reduce DC offset to within acceptable limits for the modulation that is being used. More preferably, compensation is provided by subtracting from the sampled signal, a digital value representing the DC offset in the filtered analog signal that is introduced by the digital-to-analog converter and/or the low pass filter.
Modulation systems according to the invention include a digital-to-analog converter that converts the sampled signal into an analog signal. The analog signal is filtered by a low pass filter to produce a filtered analog signal. The digital-to-analog converter and/or the low pass filter introduce DC offset into the filtered analog signal. A modulator modulates the filtered analog signal onto a carrier. A DC offset compensator compensates for the DC offset in the filtered analog signal that is introduced by the digital-to-analog converter and/or the low pass filter.
DC offset compensators according to the invention preferably include a sensor that senses the DC offset in the filtered analog signal. An analog-to-digital converter is responsive to the sensor, to convert the sensed DC offset into a digital offset signal. A subtractor is responsive to the analog-to-digital converter, to subtract the digital DC offset signal from the sampled signal, and to apply the sampled signal less (minus) the digital DC offset signal, to the digital-to-analog converter. Accordingly, the sensed offset is subtracted in the digital domain. A scaler may also be included that is responsive to the analog-to-digital converter, to scale the digital DC offset signal into a scaled digital DC offset signal. The subtractor is then responsive to the scaler, to subtract the scaled digital DC offset signal from the sampled signal.
The subtractor need not continuously sense the DC offset in the filtered analog signal, but rather may do so on an intermittent and preferably periodic basis. For example, the DC offset compensator may include a latch that is responsive to the analog-to-digital converter to intermittently latch the digital DC offset signal and to apply the latched digital DC offset signal to the subtractor, such that the latched digital DC offset signal is subtracted from the sampled signal. When the analog-to-digital converter is clocked at a first clock rate, the latch can be clocked at a second clock rate that is lower than the first clock rate.
The sensor may comprise a low pass filter that senses the DC offset in the filtered analog signal. In one embodiment, the analog-to-digital converter is a one bit delta-sigma analog-to-digital converter. In another embodiment, a polarity inverter is responsive to the sensor, to periodically invert the polarity of the sensed DC offset signal. The analog-to-digital converter converts the periodically polarity inverted sensed DC offset signal into the digital offset signal, thus reducing the effect of the internal DC offset of the analog-to-digital converter.
DC Offset compensation may be advantageously used with dual bandwidth modulators wherein the sampled signal comprises a selected one of a first digital input signal and a second digital input signal of narrower bandwidth than the first digital input signal. For example, the invention may be used with a first digital input signal that is a CDMA signal, and with a second digital input signal that is an FM signal. More specifically, the CDMA signal may be a direct sequence spread spectrum signal, and the FM signal may be an analog cellular telephone signal. The present invention may also be used in IQ modulators, also referred to as xe2x80x9cquadraphase modulatorsxe2x80x9d or xe2x80x9cquadrature modulatorsxe2x80x9d that modulate in-phase and quadrature filtered analog signals onto a carrier. Analogous modulation methods may also be provided.
Accordingly, dual-mode modulation systems and methods for a digital input signal can provide low DC offset notwithstanding the introduction of DC offset by the digital-to-analog converter and/or the low pass filters thereof. High performance and costly digital-to-analog converters need not be used. High performance off-chip low pass filters also need not be used.